The present invention relates to flame-resistant fabrics woven or knitted from yarns that are made from fiber blends.
Flame-resistant fabrics (also variously referred to as “fire-resistant”, “flame-retardant”, and “fire-retardant” fabrics) are fabrics that, once ignited, tend not to sustain a flame when the source of ignition is removed. A great deal of investigation and research has been directed toward the development and improvement of flame-resistant fabrics for use in various products such as bedding, clothing, and others. Flame-resistant clothing is often worn by workers involved in activities such as industrial manufacturing and processing, fire-fighting, electrical utility work, and other endeavors that entail a significant risk of being exposed to open flame and/or electrical arcs.
Flame-resistant fabrics include both fabrics that are treated to be flame-resistant as well as flame-resistant fabrics made from inherently flame-resistant fibers. The former types of fabrics are not themselves flame-resistant, but are made flame-resistant by applying to the fabric a chemical composition that renders the fabric resistant to flame. These types of fabrics are susceptible to losing their flame-resistance when laundered repeatedly because the flame-resistant composition tends to wash out. In contrast, inherently flame-resistant fabrics do not suffer from this drawback because they are made from fibers that are themselves flame-resistant.
Various types of inherently flame-resistant (FR) fibers have been developed, including modacrylic fibers (e.g., PROTEX® modacrylic fibers from Kaneka Corporation of Osaka, Japan), aramid fibers (e.g., NOMEX® meta-aramid fibers and KEVLAR® para-aramid fibers, both from E. I. Du Pont de Nemours and Company of Wilmington, Del.), FR rayon fibers, oxidized polyacrylonitrile fibers, and others. It is common to blend one or more types of FR staple fibers with one or more other types of non-FR staple fibers to produce a fiber blend from which yarn is spun, the yarn then being knitted or woven into fabrics for various applications. In such a fiber blend, the FR fibers render the blend flame-resistant even though some fibers in the blend may themselves be non-FR fibers, because when the FR fibers combust they release non-combustible gases that tend to displace oxygen and thereby extinguish any flame. In such blends, typically there is a greater content of FR fibers than non-FR fibers, on the theory that the flame-extinguishing ability of the FR fibers might be overwhelmed by too much non-FR fiber content.
As an example, United States Patent Application Publication US 2005/0025963 to Zhu discloses an intimate blend of staple fibers having 10 to 75 parts by weight of at least one aramid fiber, 15 to 85 parts by weight of at least one modacrylic fiber, and 5 to 30 parts by weight of at least one polyamide fiber.
Another blend of staple fibers is disclosed in United States Patent Application Publication US 2004/0192134 to Gibson et al. The blend includes at least about 60 percent FR fibers (modacrylic and/or aramid) and up to 40 percent synthetic or natural non-FR fibers such as cotton or wool.
U.S. Pat. No. 6,787,228 to Campbell et al. discloses a yarn formed of a blend of fibers including at least about 70 percent modacrylic fibers combined with at least about 3 percent high-performance, high-energy-absorptive fibers such as aramid.
In the United States, it is desirable and often required for clothing worn by certain types of workers to pass a dual-hazard performance specification encompassing both the flame-resistance standard F1506 of the American Society for Testing and Materials (ASTM) as well as the flash fire protection standard of NFPA 2112-2012. The ASTM F1506 standard, entitled “Standard Performance Specification for Flame Resistant Textiles Materials for Wearing Apparel for Use by Electrical Workers Exposed to Momentary Electrical Arc and Related Thermal Hazards”, sets various standard performance specifications for a fabric, among which are specifications for the ability of the fabric to self-extinguish after being ignited. When the ignition source is removed, the fabric must self-extinguish in less than 2 seconds and have less than a 4-inch char length according to ASTM Test Method D6413 (“Standard Test Method for Flame Resistance of Textiles”, also referred to as the Vertical Flame test). The F1506 performance standard also includes standard test ASTM 1959 (“Standard Test Method for Determining the Arc Thermal Performance Value of Materials for Clothing”), which measures the level of protection that the fabric provides against electrical arc exposure. The ASTM 1959 test establishes four levels of electrical arc protection as measured by the fabric's Arc Thermal Performance Value (ATPV), expressed in cal/cm2: Level I is 4 cal/cm2; Level II is 8 cal/cm2; Level III is 25 cal/cm2; Level IV is 40 cal/cm2. At least Level II certification (ATPV greater than 8.0 cal/cm2) is required for clothing worn by many electrical utility workers. ASTM F1506 also has minimum performance specifications for tensile breaking strength (40 pounds) and tear-resistance (4.0 pounds) of the fabric under standard test conditions, as well as a maximum allowable 3% shrinkage in both the warp and fill directions.
The NFPA 2112-2012 specification's notable requirements include a maximum 4-inch char length (both before and after 100 industrial launderings) and a maximum 10% thermal shrinkage.
In addition to the above-noted performance specifications of fabrics, other properties are also important if a fabric is to be practical and commercially viable, particularly for clothing. For instance, the fabric should be durable under repeated industrial launderings and should have good abrasion-resistance. Furthermore, the fabric should be readily dyeable to dark, solid shades of color, and should be comfortable to wear.